Micro ElectroMechanical Systems (MEMS) are used in a variety of applications from navigational or guidance systems to ballistic applications. In the manufacture of some MEMS devices, a MEMS wafer is attached to a package or board (e.g., substrate), for example, using flip chip methodologies such as ball grid array (BGA), controlled areas of solder, gold stud bump flip chip, etc., but without actually flipping the chip. Further, the MEMS wafers are mounted face up instead of face down on the substrate, but still require the mechanical advantages of flip chip mounting. The MEMS wafer typically includes a glass layer coupled to a continuous metallic layer with a number of MEMS components attached to the free or front surface of the glass layer. Once the MEMS wafer is produced, it is diced or segmented to make individual dies. Each die may then be attached to the substrate by applying a bump pattern to the substrate and then bonding the die to the bump pattern. The metallic layer is generally used solely as a layer to adhere the MEMS die to a next level of packaging (i.e., the substrate). The alignment of the die relative to the substrate affects the performance and accuracy of the MEMS device.
One conventional method for making the MEMS wafer, dicing the wafer, and attaching the dies to a substrate includes (1) dicing the wafer using alignment marks placed on the metallic layer of the wafer; (2) applying a bump patter to the substrate; (3) aligning a die on a portion of the substrate using the edges of the die; and (4) bonding the die to the substrate. In each of these steps, an amount of error may be introduced and these errors may accumulate. For example, the position of each MEMS component in relation to the edges of each respective die may vary slightly because of errors introduced during the dicing operation, errors introduced when applying the bump pattern to the substrate, errors introduced when aligning the die to the portion of the substrate and errors introduced when bonding the die to the substrate.
One approach to correcting one or more of the above-identified alignment errors is to place the bump pattern on the backside of the die to reduce any misalignment of the die to the bump pattern. This approach works only if there is no metallic layer forming the backside of the die. However, when the backside of the die is a metallic layer or a non-transparent layer there is no way to sufficiently align the MEMS component to the bump pattern and thus alignment errors still persist.